An oil pump for an automotive vehicle is normally a standard positive-displacement gear pump that is directly and permanently driven by the engine drive shaft. The pump has an intake connected to a sump formed by the engine oil pan and an output that leads to all of the various joints to be lubricated, which joints are set up so that any excess lubricant flow will run back into the sump.
Such an arrangement has the advantage of great simplicity, as a gear pump can be counted on to give relatively troublefree performance for years. In addition such a pump will even get the lubricant circulating when it is very viscous in very cold weather, providing lubrication to the engine even on cold starts.
Nonetheless the average engine normally only needs a certain maximum lubricant flow, measured in volume with respect to time (e.g. liters/minute). The flow through the positive-displacement gear pump, however, is directly proportional to its drive speed, typically in revolutions per unit of time, and is not responsive to the engine's need of lubricant or the viscosity of the lubricant, which is largely a function of its temperature. Thus at high engine speeds in cold starts the pump can produce dangerously high pressures, and at low speeds in cold starts the pressure can be inadequate for proper lubrication.
It is therefore standard to provide a pressure-limiting valve that is connected between the output of the pump and the sump and that opens above a predetermined pressure, normally about 5bar, to prevent this pressure from damaging any hoses or the like in the lubricating system. The valve curve can closely correspond to the engine's need for lubricant which increases slightly, but generally asymptotically, with increasing engine speed.
Although such a solution has the advantage of simplicity, preventing overpressures that would be caused by using a constant-displacement pump, it is wasteful of energy. The extra pump capacity is squandered, its mechanical energy being converted into heat that must be dissipated. In these energy-conscious times such inefficient energy use constitutes a distinct disadvantage of this system.
Accordingly, U.S. Pat. No. 4,342,545 describes a vane pump used in an automatic transmission to keep pressures steady even when drive speed varies. This pump comprises a stator or housing having an outer part forming intake and output ports and an inner part pivotal about a pivot axis on the outer part and defining a chamber centered on a stator axis extending between the ports. These ports open axially into the chamber at locations diametrally offset relative to the axis and the chamber is closed radially outward relative to the axis by the inner part. A rotor has a body rotatable in the chamber about a rotor axis parallel to and normally offset from the stator axis and having an outer surface centered on the rotor axis. The rotor body is formed with a plurality of angularly equispaced and radially extending guides that open at the outer rotor surface. The rotor axis does not move relative to the stator, but the stator axis defined by the inner stator part can move from an inner position coinciding with the rotor axis to an outer one parallel to the offset therefrom. Respective vanes of substantially the same radial length are received in the guides and have radially outer ends riding on the inner stator surface and radially opposite inner ends. At least one circular inner ring on the body has an outer ring surface radially outwardly engaging the inner vane ends. These vanes angularly define compartments axially closed by the stator outer part, radially outwardly closed by the stator inner part, and radially inwardly closed by the inner ring.
Thus when the stator axis is offset in one direction from the rotor axis and the rotor is rotated in a predetermined sense, the compartments move from the intake to the output port and decrease in volume at the output port. A spring-type biasing unit urges the inner housing part pivotally about the pivot axis in the one direction increasing compartment size with a fixed force and a hydraulic biasing unit is pressurized from the outlet port to urge the inner housing part pivotally in the opposite direction decreasing compartment size.
With this system, therefore, as the pressure at the output port increases, the inner housing part is pivoted inward to bring the rotor and stator axes together, and thereby reduce the eccentricity of the inner stator surface relative to the rotor axis and concomitantly reduce the pumping volume. The pump capacity is therefore reduced above a certain output or back pressure, making the pump highly efficient in that it only moves enough liquid to produce a predetermined pressure.
Such a pump can only work where the viscosity of the liquid does not vary appreciably. If it were to be used in place of the standard engine gear pump, in cold weather the backpressure created by the highly viscous engine oil would be sufficient to set the pump at a very low or zero displacement. This would result in an excessive reduction in lubricant flow and could lead to damage to the joints in the engine. These joints will heat up and need lubricating rapidly, before the oil in the various feed lines warms up correspondingly and the pump can achieve some meaningful flow with the thinning liquid lubricant. Thus this type of pump is unsuitable for use as an oil pump of an automotive vehicle whose lubricant varies in viscosity and whose pump drive speed in turn varies independently of the engine's need of lubrication or of the viscosity of the oil.